1. Field of the Invention
The present invention relates to a magnetization state control device and a magnetic information recording device that controls a magnetic state of a magnetization material by current drive.
2. Description of the Related Art
In recent years, various information recording elements that operate based on novel operation principles have been developed. In particular, technical fields pertaining to various elements that utilize spin characteristics of electrons have been receiving attention as new development in the field of electronics and eventually established as a new independent field, which is referred to as spintronics. In particular, studies of magnetization reversal in a magnetic material element caused by injecting a spin-polarized current have been attracting much attention for a variety of possible applications.
The technology of magnetization reversal by spin-polarized current is gathering attention for the following reason. In the case of methods of magnetization reversal using an external magnetic field like those used in the existing magneto-resistive random access memory (MRAM), miniaturization of the element leads to an increase in the intensity of the magnetic field required to reverse the magnetization. In addition, the magnetic field exhibits a long range interaction by its nature. In view of these, it is considered that increasing the capacity of the existing MRAM while using an external magnetic field as means for creating a reversely magnetized region will reach the limit sooner or later.
For the above described reason, magnetization reversal by spin-polarized current injection have been intensively studied as a method that can remove the aforementioned limit. In the late 1990s, magnetization reversal at low temperatures in a three-layered film was confirmed by experiments (see E. B. Myers et al., “Current-Induced Switching of Domains in Magnetic Multilayer Devices”, Science. Vol. 285, (1999) 867, (non-patent literature 1)), which is followed by confirmation of magnetization reversal at room temperatures in a giant magneto-resistance (GMR) film in the 2000s (see J. A. Katine et al., “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars”, Phys. Rev. Lett. Vol. 84, (2000) 3149 (non-patent literature 2)). Thus, studies aiming at practical implementation of this technology have been intensively conducted mainly in the Europe and the United States. However, with increases in the degree of integration, problems such as increased complexity of the element structure are beginning to be encountered.
In the following, an information recording method and reading method using a uniformly magnetized structure/reversely magnetized structure will be described taking a MRAM as an example.
<Writing of Information>
A TMR (Tunneling Magneto-Resistance) element that constitutes a MRAM has a three-layer structure in which an insulator thin film having a thickness of a few layers of atoms is sandwiched between two ferromagnetic thin films. In this device, while the direction of the electron spin in one of the ferromagnetic thin films is fixed, the direction of electron spin in the other ferromagnetic film can be changed by a magnetic field that is externally applied thereto. In the device, the state in which the electron spins in the both films are parallel is associated, for example, with “0”, and the state in which the electron spins are antiparallel is associated with “1”. Thus, information can be recorded by controlling the direction of the electron spin (i.e. the magnetization state) in the ferromagnetic thin film that is not fixed.
<Reading of Information>
Reading of information in the MRAM is performed utilizing the characteristic of the TMR element that its electric resistance changes depending on the difference in the directions of electron spins. When the directions of the electron spins in the two ferromagnetic thin films in the TMR element are parallel, the resistance of the TMR becomes relatively small. In contrast, when the directions of the electron spins are antiparallel, the resistance becomes relatively large. Therefore, the spin state of the TMR element can be known by detecting the change in the value of the resistance.
The address access time of the MRAM is 10 to 20 ns and the cycle time thereof is 20 to 30 ns, which are approximately five times those of the DRAM (Dynamic Random Access Memory). Thus, high speed writing and reading on par with SRAM (Static Random Access Memory) can be achieved. In addition, the MRAM is advantageous in that the power consumption is as small as approximately one-tenth that of the Flash memory, and that it can be integrated to a high degree.
Recently, a concept of a racetrack type magnetization information recording device (or racetrack random access memory) was made public (see U.S. Pat. No. 6,834,005 (patent literature 1) and “Spintronics Devices Research: Magnetic Racetrack Memory Project”, IBM, available online at the URL of <http://www.almaden.ibm.com/spinaps/research/sd/?racetr ack> (non-patent literature 3)). In this device, a magnetic material that is large enough to contain a plurality of magnetic domain structures is prepared as an information recording section (in the form of, for example, a tape) instead of handling a single ferromagnetic structure as a single memory element, and reversely magnetized states are written in the information recording section. This is based on a concept similar to the magnetic tape, but what is fundamentally different is that unlike with recording/reading using a magnetic tape in which a recording head or the magnetic tape is moved to make access to adjacent recorded information, the reading and recording portions are mechanically fixed in this device, and a magnetization state recorded in the magnetic material is driven by a spin-polarized current to move the position of a magnetic domain so that it is subjected to a recording/reading operation. This is based on a completely novel concept that provides a possibility that a magnetic domain structure as it is in a ferromagnetic material can be used as a bit, which is different from the concept of conventional devices in which each element corresponding to one bit is mechanically fixed and information recording/erasing is individually performed on each of them. Thus, the size corresponding to the distance between adjacent elements in conventional devices is decreased to the size of adjacent magnetic structures, whereby ultimate size reduction is achieved. Therefore, an increase in the integration density and resultant performance of high calculation speed are expected.
The present invention has been made based on experiments for generating a reversely magnetized region by current pulse supply to a ferromagnetic material in a weak magnetic field that were performed based on the above described background (see Y. Togawa et al., “Current-Excited Magnetization Dynamics in Narrow Ferromagnetic Wires”, Japan. J. Appl. Phys. Vol. 45, (2006) L683 (non-patent literature 4) and Y. Togawa et al., “Domain Nucleation and Annihilation in Uniformly Magnetized State Under Current Pulses in Narrow Ferromagnetic Wires”, Japan. J. Appl. Phys. Vol. 45, (2006) L1322 (non-patent literature 5)).
In conventional magnetic information recording element such as MRAM, information is recorded as a state of magnetization formed in a ferromagnetic material in the element. To control the magnetization state, a magnetic field is applied externally to the element, whereby reversal of magnetization and formation of multiple magnetic domain structures etc. are caused reflecting the intensity of the magnetic field. In the case where magnetization reversal is to be caused by applying a magnetic field, miniaturization of the element requires an increase in the intensity of the magnetic field to be applied. Thus, we face the technical difficulty of applying an intense magnetic field to a very small region. This has been one of the obstacles to high density integration.
Furthermore, since the effect of magnetic field reaches infinity by its nature, if the integration density is made high, there is a possibility that a magnetic field generated to control a certain element disturbs the magnetization state of an adjacent element. This has also been another obstacle to high density integration.
In contrast, in the method using spin-polarized current injection, these problems can be eliminated. For example, Japanese Patent Application Laid-open No. 2006-196708 (patent literature 2) teaches to increase the amount of electric current supplied to a magnetic information memory element beyond a certain critical electric current amount by arranging the shape and material of the element. Thus, a method of recording/erasing information using a magnetic domain structure (or magnetization state) spontaneously formed upon current supply has been developed.
In the method using spin-polarized current, however, it is considered to be difficult to control the magnetization state reliably. In particular, in the operation of actual elements, there is a possibility that both the information recording operation and information erasing operation become unstable, because these operations depend only on control of the amount of electric current to be supplied.